Supply the correct pronoun:
Sony and Rony have a house. _____ house is quite big.

Here, “Their” is used because it refers to Arun and Lata’s house (here, the house is their possession) - which is a plural subject.
For possession of a singular subject, we use His/Her.
“Theirs” is used when the object is plural.
Example: It is my house, the left one is theirs.